The present invention relates generally to a method of and apparatus for filling a succession of containers. It is particularly, but not exclusively, of application in filling kegs on a keg filling line.
In the brewing industry, kegs are usually filled with beer or lager using a piece of equipment commonly called a racker. Empty kegs are positioned successively at one end of the racker and pass sequentially through a series of stations or heads where different operations are performed on the kegs. Typically the kegs pass through a plurality of cleaning heads and a steam sterilizing head before they arrive at a filling head where the kegs are filled. Movement of the kegs between the heads is usually by means of a walking beam conveyor. After filling, the kegs move to a discharge platform where they are removed from the machine.
Obviously it is important that each keg is filled with the correct amount of beer or lager. In fact average contents legislation is likely to lay down that a keg must contain at least 98% of its nominal contents. Over filling, on the other hand, can be very costly to the beer producer in some countries excise duty is payable in respect of the excess beer in kegs which is not payed for by the customer. It is difficult to achieve sufficient accuracy with existing filling arrangements.
Several different filling systems have already been used or have been proposed.
In one system the volume of beer flowing into a keg is measured using a volumetric flowmeter. However, flowmeters per se may not be particularly accurate, and further inaccuracies are introduced into such a system by other components such as pipework and valve closures. This calls for painstaking and frequent recalibration of the apparatus.
In another proposed system, the gross (filled) weight of a keg is measured after filling and the contents calculated by subtracting a nominal tare (empty) weight of the keg from this figure. This approach however is very inaccurate since the weight of kegs having, in theory, the same nominal tare weight may vary by as much as +2 kg, due to non standardized production, damage during use (some kegs may be in use for 25 or more years) and replacement of parts such as the valve mechanism forming part of the keg.
A more recent system has proposed that each keg is weighed individually at the filling head. The empty weight of the keg is first taken and the weighing system then zeroed. The keg is then filled until a predetermined weight, corresponding to the desired weight of the contents is reached. The filling is then stopped.
There are several problems associated with such a system. Firstly, the keg is only at the filling head for a relatively short period of time (typically 60 seconds) during which the keg must be weighed empty and then filled. This does not allow much time for an accurate measurement of the tare weight of the keg. This approach inevitably leads to inaccuracies in the zeroing of the system and thus in the total contents. Secondly, the keg is filled relatively quickly and significant kinetic energy and movement is generated on the weighing device, which leads to inaccurate weight measurement during filling. Thirdly, the weighing device usually employed is a platform floating on tension wires, the weight being calculated from the tension in these wires. In such systems, however, the weighing and filling station is immediately adjacent a preceding steam sterilization station, with the result that the tension wires may be subjected to blasts of high temperature steam. This approach of course adversely affects the accuracy of the weight measurement and thus the contents.